Most conventional heat pumps, whether used for heating or cooling purposes, utilize a refrigerant having suitable thermodynamic properties such as ammonia or certain organic fluids, mainly freons. Basically such heat pumps consist of a closed system comprising an evaporator, a compressor, a condensor, if necessary an expansion valve, and various controls. The working fluid (refrigerant) evaporates in the evaporator at a low temperature and pressure, extracting from the surroundings a quantity of heat equal to its heat of vaporization. The refrigerant vapors are compressed by the compressor to a pressure and temperature sufficiently high to enable the refrigerant to condense in the condenser by giving up its heat or vaporization to a stream of cooling water or to the atmosphere.
Heat pumps using water as the refrigerant have also been proposed (see for example U.S. Pat. No. 4,003,213 and Israel Patent 64871) and such systems include ejectors, absorption systems and mechanical vapor compression (MVC) systems. The use of water as a refrigerant is thermodynamically desirable owing to its good thermophysical properties and the advantages of employing direct contact heat transfer, eliminating the need for costly and thermodynamically inefficient heat exchangers. Furthermore, water is the most "environmentally friendly" working fluid available, in contrast with currently used organic working fluids (CFCs) which are environmentally damaging and are likely to be restricted or banned altogether in the coming decade.
Known heat pumps employing water as a working fluid of the ejector and absorption system types are characterized by low efficiency, whereas MVC systems have a much higher efficiency, typically about 2 to 3 times greater. However, a major difficulty involved in the use of water as a refrigerant in MVC systems is the very high specific volume of water vapor which requires the use of a very large compressor. Thus, in a large size refrigeration heat pump having a cooling capacity of about 3 to 10 MW, the required flow rate of water vapor would be about 300-400 m.sup.3 /sec which is considered a relatively high volumetric flow rate. In addition, for a 20.degree.-30.degree. C. temperature difference (between the space to be cooled and the ambient temperature of the air or cooling water) a compression ratio (CR) of the order of 1:9 would be required.
For this range of flowrates and compression ratios, two basic compressor types are suitable, namely axial and centrifugal. The axial type, as used mainly on aircraft engines is well developed and has a high efficiency but is expensive to produce, therefore the centrifugal type is the most promising for this application. However, to date no centrifugal compressors have been developed which come even close to fulfilling the target specification (i.e. 300-400 m.sup.3 /sec., 1:9 CR) mentioned above. Compression ratio is a function of tip speed. Typical tip speeds found on small aluminum compressors are of the order of 500 m/sec which gives a CR of approximately 1:3.
Conventional large diameter compressors, which in general do not go beyond 1.6-1.7 m impeller diameter are mainly made of fabricated steel construction (Aluminum alloy casting or machining from solid as used on smaller machines, is not practical in the larger sizes due to difficulties in cooling massive metal sections). Fabricated steel construction generally involves welding individual cast steel blades onto a solid cone (e.g. see Allis Chalmers, Catalogue, 1980, page 337). Such designs are not capable of sustaining the mechanical loads found at say 500 m/sec tip speed due to stress limitations on welded sections. Hence the tip speeds attained by such designs are generally quite low, resulting in compression ratios not more than approximately 1:1.6. This severely restricts the range of process applications. In addition, the sheer weight of the rotor resulting from such construction methods entails a complicated and expensive rotor support system.
Thus, to summarize, conventional large centrifugal compressors exhibit limited CR and volumetric capacity and are costly to manufacture.